Optical fibers are used in a wide array of fields, for example, to transmit information in the context of telecommunications and to act as a sensing medium in the context of sensors, such as oil well temperature sensing. In many applications, there is a need to send a light signal that is propagating in an optical fiber around a tight bend. For instance, the optical fiber may be formed to include a substantially 180° turnaround. In order to form the turnaround, a great deal of stress is placed on the bent fiber section. Between the stress of bending the fiber and the bend itself, the fiber often suffers from material integrity and optical loss issues.
To, at least, partially, address the issue of material integrity, the bent fiber section is often packaged or otherwise encased to prevent subsequent contact that could further weaken the fiber. However, the packaging or encasing process often occurs at high temperatures, which compounds the issues of material integrity and optical loss. In addition, the fiber may be utilized in an environment in which high operating temperatures further compound the issues of material integrity and optical loss, and degrade the operating life of the sensing fiber.
One approach known in the art uses low temperature fiber optic components and, thereby, avoids high temperatures during both manufacturing and operation. For instance, according to U.S. Pat. No. 7,269,320, which is directed to AFL Telecommunication's “Mini-Bend” device, the fiber is stretched under low-to-moderate heat to reduce the diameter of the glass structure of the core of the fiber, bent to form the turnaround and potted to protect the bent fiber section. However, the low temperature fiber optic approach has a limited operating range, the packaging (i.e., the potting) is fragile and the fiber, thus, remains sensitive to shock and vibration.
Another solution places a splice box at the end of a dual-fiber fiber optic cable in order to accommodate a splice between the two fibers to create the loop. The splice box is, typically, a mechanical package that is much larger than the cable creating mechanical interference issues when the cable is installed downhole, such as in an oil well. The splice box also requires a high temperature pressure seal to block the ingress of fluids and gases in the downhole environment.
Another solution bends an optical fiber and anneals the bent optical fiber to remove strain caused by the bending process. For instance, U.S. Pat. No. 4,812,001 discloses a method of bending an optical fiber against a cylindrically-shaped cartridge heater to form a 90° elbow-shaped turn. The temperature of the heater is elevated to simultaneously soften and annealed the glass (i.e., within seconds or minutes). However, the disclosed method is limited to making approximately 90° turns. In addition the heater only anneals the glass immediately proximate the bending surface of the heater and does not address strain at the remote ends of the optical fiber.
The object of the present invention is, therefore, to provide a sensing cable with a turnaround, which, among other desirable attributes, significantly reduces or overcomes the above-mentioned deficiencies of prior sensing cables.